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Thread: About dance stacks..

  1. #21
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    Quote Originally Posted by efinque View Post
    Isn't the power (edit : by power I mean the overall volume in dB) measured from 1m distance in watts (in a vacuum?) I'm fully aware of the floor reflection thing..
    I'm not sure how they measure sound volume in a vacuum. Do you mean open space/whole space as in no walls resulting in reflections?

  2. #22
    Quote Originally Posted by mitchiemasha View Post
    Imagine a basket throwing itself forward as the cab has backwards movement, reaching the limits of the speaker earlier, the resistance in suspension, overshooting the magnet, bottoming out, with less air been moved.
    Baskets don't throw themselves forward. Cones do. In the real world this scenario does not occur.
    Bill Fitzmaurice
    Bill Fitzmaurice Loudspeaker Design
    www.billfitzmaurice.info/forum

  3. #23
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    Quote Originally Posted by Bill Fitzmaurice View Post
    Baskets don't throw themselves forward. Cones do.
    oh, wow... Yes... that is an obvious correction but helps little. To be fair, cones don't throw them selves forward either. Now we could delve into a debate on if it's the magnet, the electrical current or the coil that throws the cone forward but the specifics of that are purely a distraction to the issue i'd addressed.

    In the real world this scenario does not occur.
    Regardless to my term used the scenario does occur and i'm not one to simply be told... I need to understand what is happening to put my mind at rest. I can't simply be told the answer i need to understand the answer. Do you care to share any science on the subject.

    Why does it not happen?

    I'm also a bit curious to why that was the only point you picked up on since your last post. Yet shared no insight to expand knowledge.
    Last edited by mitchiemasha; 03-10-2017 at 10:47 AM.

  4. #24
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    Quote Originally Posted by Bill Fitzmaurice View Post
    Baskets don't throw themselves forward. Cones do. In the real world this scenario does not occur.
    You would have to hit the resonant frequency of the hang to get any back and forth movement of the stack, which would be way lower in frequency than that of music, probably less than 1 or 2 hz. One place it can make a difference is in large arrays of infinite baffle mounted home theater drivers mounted in an attic. Because home theater can go down to single digits in frequency, the timber structure of your house can start to oscillate. There a manifold with the drivers facing opposite directions can counter this.

  5. #25
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    Quote Originally Posted by Coronaoperator View Post
    You would have to hit the resonant frequency of the hang to get any back and forth movement of the stack.
    This would be a nightmare and a whole new can of worms. I'm not on about at that scale. The friction of the weight of the speaker on the floor is removed so surely there would be more backward movement to the cones forward movement. Only slight but still some. What effect would this have on the sound? Obviously good systems will anchor the speaker adequately. With them being at a bottom of a hang potentially adding more problems to this issue.

  6. #26
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    The more i think about it... it's the offset in the time domain in the movement that would cause more of an issue to the sound. I'm still refreshing my mind/reading up on Newtons 3rd Law to rest my thoughts.

  7. #27
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    Quote Originally Posted by mitchiemasha View Post
    I need to understand what is happening to put my mind at rest. I can't simply be told the answer i need to understand the answer. Do you care to share any science on the subject.
    Well for sure newtons 3rd law holds true. That cone moving back and forth exerts an equal and opposite force on the cabinet which on a hang will move the cabinet back and forth but how much?

    force=mass x acceleration or f=ma

    so using newtons 3rd law;

    mass(cabinet) x acceleration (cabinet) = mass(cones) x acceleration (cones)

    Assuming the mass of the cones of a double 18" cabinet is ~210 grams each so mass(cones) = 420 grams for the both of them or mass(cones) = 0.42 kg

    and acceleration = 2 x distance / time squared

    lets find acceleration of the cones at 60 hz:

    time @ 60 hz = 1/60 = 0.0167 seconds
    distance of cone travel (x-max) lets assume full bore is 18mm = 0.018 meters

    acceleration (cones) = 2 x 0.018 m / (0.0167 s)squared
    acceleration(cones) = 129 m/s2

    assume the mass(cabinet) = 112 kg

    back to newtons 3rd law:
    mass(cabinet) x acceleration (cabinet) = mass(cones) x acceleration (cones)
    112 kg x acceleration (cabinet) = 0.42 kg x 129 m/s2

    gives us acceleration (cabinet) = 0.484 m/s2

    now distance the cabinet moves d = 1/2at2
    d= 1/2 x 0.484 m/s2 x 0.0167 s x 0.0167 s
    d= 0.0000674 m
    d = 0.067 mm

    So there you have it, hanging in free space with a 60 hz tone playing at full balls to the wall 18mm x-max the cabinets are vibrating back and forth 0.067 mm or less than 3 thousands of an inch. So like Bill said, doesn't make a lick of difference in the real world. Once you factor in that the cabinet isn't floating in free space and actually is constrained by the shackles or whatever is holding it up, the movement would be even less.

    Disclaimer: I never took into effect the mass of the air that is also being moved by the cones which is about 80 g per cubic foot so depending on that the effective mass of the cones could be increased by a small amount, perhaps even doubled. Still doesn't make a difference.
    Last edited by Coronaoperator; 03-10-2017 at 12:23 PM. Reason: clarify units, added mass of air

  8. #28
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    Quote Originally Posted by mitchiemasha View Post
    The more i think about it... it's the offset in the time domain in the movement that would cause more of an issue to the sound.
    velocity = acceleration x time
    v = 0.484 m/s2 x 0.0167 seconds
    v = 0.008 m/s, thats the max velocity that the cabinet will achieve in free space.

    which compared to the speed of sound at 343 m/s means that any Doppler effect won't even be measurable at those low frequencies never mind noticeable. ie, instead of hearing a 60 hz tone, you will hear a tone that warbles between 59.999 hz and 60.001 hz.
    Last edited by Coronaoperator; 03-10-2017 at 11:58 AM.

  9. #29
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    Now to add a whole new level of complexity to the equation, in a ported box the cones are not the only thing with mass accelerating. The mass of the air in the ports are too and they are out of phase with the cone! That means that at frequencies that the port is tuned for that acceleration of mass of the air would counteract the acceleration of the cone. Depending on the velocity that the air in the ports achieve, they may have more of an effect on the acceleration of the cabinet than the cones! Just food for thought.

  10. #30
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    See, now we are talking. Thank you. Some serious maths. 0.067 does seem incredibly low, a lot lower than i imagined. I'm sure I've seen speakers wobble themselves about before but something else must of been at play. Don't forget my initial comment was referencing putting a bass bin on a pole (because flying is supposedly better) not a very expensive line array that most of us could never afford.

    What is 0.067mm expressed as a % of 18mm and how would that relate to a db loss?

    I'm currently reading up on inertia, as underestimating that might of had more to do with overestimating the factors of ground friction as the result of gravity been more beneficial for the speaker.
    Last edited by mitchiemasha; 03-10-2017 at 01:41 PM.

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